Mapping variations of a surface

ABSTRACT

A method for characterizing a surface, consisting of dividing the surface into pixels which are characterized by a parameter variation and defining blocks of the surface as respective groups of the pixels. The pixels are irradiated in multiple scans over the surface with radiation having different first polarization states. At least part of the radiation returning from the pixels is analyzed using second polarization states, to generate processed returning radiation. For each scan, block signatures of the blocks are constructed using the processed returning radiation from the group of pixels in each block. Also for each scan, a block signature variation is determined using the respective block signatures of the blocks, and, in response to the block signature variation, one of the first polarization states and at least one of the second polarization states are selected for use in subsequent examination of a test object.

FIELD OF THE INVENTION

The present invention relates generally to quality inspection, andspecifically to high throughput inspection.

BACKGROUND OF THE INVENTION

In a wafer fabrication facility, scanning using an optical system is oneof the recognized methods for inspecting wafers for defects,irregularities, etc. The scanning approach irradiates all or a specificregion of the wafer, for instance in a die or cell on the wafer, andmeasures one or more parameters of the returning radiation, which may bescattered, diffracted, and/or reflected radiation. The measuredparameters may be compared with other respective, assumed “standard”parameters, typically in a cell-cell or die-die comparison, or in acomparison against previously determined values, to determine if theirradiated region is within specification.

The smallest features of elements of present day wafers typically havedimensions of the order of tens of nanometers. Systems forcharacterizing wafers at these orders of magnitude are known in the art,for example scanning electron microscopes (SEMs), scanning X-raymicroscopes (SXMs), atomic force microscopes (AFMs), and OpticalCritical Dimension (OCD) tools. However the scan rate of such systems istypically extremely low so that they are usually only used tocharacterize or inspect relatively small fractions of a wafer. If theyare used on a whole 300 mm wafer, the procedure takes many hours ordays.

U.S. Pat. No. 6,862,491 to Levin et al., whose disclosure isincorporated herein by reference, describes a method to extend theprocess monitoring capabilities of a semiconductor wafer opticalinspection system so as to be able to detect low-resolution effects ofprocess variations over the surface of a wafer at much highersensitivity than heretofore possible.

A paper titled “Novel inspection technology for half pitch 55 nm andbelow” by Omori et al, in Metrology, Inspection, and Process Control forMicrolithography XIX, edited by Richard M. Silver, in Proc. of SPIE Vol.5752, 2005 is incorporated herein by reference. The paper relates to asystem for inspecting surfaces.

The following U.S. patents and patent applications, all of which areincorporated herein by reference, relate to systems for inspectingsurfaces: U.S. Pat. Nos. 6,512,578, 6,693,293, 7,027,145, 7,248,354,7,298,471, 7,369,224, 7,372,557, 2004/0239918, 2006/0098189,2006/0192953, 2006/0232769, 2007/0046931, 2008/0094628.

However, notwithstanding the systems at present available, an improvedmethod for inspecting surfaces is desirable.

SUMMARY OF THE INVENTION

In an embodiment of the present invention, a state of incident polarizedradiation to be used to irradiate a surface of objects, typically waferssuch as production wafers, is determined. The state of incidentpolarized radiation is also herein termed a polarization optics settingor a polarizer setting. For the polarizer setting, corresponding one ormore polarization states applied by respective analyzers to radiationreturning from the surface, herein also termed analyzer optics settingsor analyzer settings, are determined. The polarizer and analyzersettings are selected so that the combination of the two settingsprovides an optimal or maximum sensitivity for parameter variations onthe surface. The polarization states for the polarizer and analyzersettings may comprise linear, circular, and/or elliptical polarizedradiation.

In one embodiment a physical reference wafer is first characterized,typically by measuring a parameter-under-investigation, such as a linewidth, with a system such as a scanning electron microscope (SEM), sogenerating reference measurements of the parameter-under-investigation.The reference wafer is then scanned multiple times with different statesof irradiating polarized radiation, each respective scan comprisingfocusing a spot onto the surface and scanning the spot across thesurface. The spot defines a size of pixels into which the surface isdivided. Typically there are 3-4 pixels per spot, but other numbers ofpixels are possible. The different states of incident polarizedradiation comprise differently oriented linearly polarized radiation,left and right circularly polarized radiation, and left and rightelliptically polarized radiation having different eccentricities.

Returning radiation from the spot may be detected using bright field,and/or gray field, and/or dark field detectors, and polarizing analyzersare positioned in front of at least some of the detectors. Therespective analyzer settings may all correspond to the incidentpolarizing radiation state, may all be different from the incidentpolarizing radiation state, or some settings may be the same and somesettings may be different. A processor stores measurements made by thedetectors, derived from the radiation returning from pixels on thesurface of the reference wafer.

The processor groups the pixels into blocks, typically rectangularblocks having sides of the order of 50 pixels, and calculates arespective block signature for each of the blocks.

In a disclosed embodiment a block signature is a function of thereturning radiation measurements of the one or more detectors, of allthe pixels in the block, for one given state of incident polarizedradiation.

As an example of a block signature, assume that the incident radiationis linearly polarized, that returning bright field radiation and grayfield radiation are detected, and that an analyzer that is setorthogonal to the incident polarization is in front of the gray fielddetector. The block signature may be an arithmetic mean of the brightfield and gray field measurements of all the pixels of the block.

The processor compares block signatures, produced by the differentstates of incident polarized radiation and respective different analyzersettings, with the reference measurements. From the comparisons, theprocessor selects the polarizer setting and analyzer settings thatproduce the highest block signature change, in absolute terms, for thechanges of the reference measurements, i.e., for which the blocksignatures have a maximum sensitivity to changes in the referencemeasurements. The processor uses the selected polarizer setting andanalyzer settings in examining or scanning test objects, such asproduction, calibration, or research and development wafers, and usesthe function to generate block signatures of the objects. The blocksignatures may characterize the objects in the same manner, and withgenerally the same resolution, as the reference measurements, but a scanto produce the block signatures takes a fraction of the time required toform the reference measurements for the objects.

In an alternative embodiment, a composite block signature is a generatedfunction of the returning radiation measurements of all the pixels inthe block for two or more different states of incident polarizedradiation. For each different incident state of polarized radiationthere is a respective set of analyzer settings.

For example, a first incident polarization state may be linear andparallel to a y-axis of a test object, and detected radiation may befrom a bright field detector and a dark field detector. An analyzer infront of the bright field detector may be linear, set to correspond tothe incident radiation, and an analyzer in front of the dark fielddetector may be set to be right circularly polarized. A second incidentpolarization state may be linear and parallel to the x-axis of a testobject. For the second polarization state the bright field analyzer maybe linear and orthogonal to the incident radiation, and the dark fieldanalyzer may be set to be left circularly polarized. The composite blocksignature may be an overall mean of a first multiple of the average ofthe bright and dark field measurements from the first incident radiationand a second multiple of the average of the bright and dark fieldmeasurements from the second incident radiation.

The processor compares different composite block signatures with thereference measurements. From the comparisons, the processor selects thecomposite block signature change that has a maximum sensitivity tochanges of the reference measurements. To examine objects, the processoruses data from scans of the objects with the states of incidentpolarized radiation and analyzer settings required for the function thatgenerates the selected composite block signature, and generatescomposite block signatures of the objects using the function.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings, a brief description of which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a surface examination apparatus,according to an embodiment of the present invention;

FIG. 2 is a schematic diagram of a surface, according to an embodimentof the present invention;

FIG. 3 is a flowchart describing a process to determine a polarizationstate to be used in investigating a production wafer, according to anembodiment of the present invention;

FIG. 4 shows tables formed by a processor in performing the process ofFIG. 3, according to embodiments of the present invention;

FIG. 5 is a flowchart describing a process to determine one or morefunctions of polarization states to be used in investigating aproduction wafer, according to embodiments of the present invention;

FIG. 6 shows tables formed by the processor in performing the process ofFIG. 5, according to embodiments of the present invention;

FIG. 7 shows graphs of reflectivity vs. line width, for two modes oflinearly polarized radiation, according to an embodiment of the presentinvention;

FIG. 8 shows exemplary results obtained using the process of theflowchart of FIG. 5, according to an embodiment of the presentinvention;

FIG. 9 is a flowchart describing a process to determine polarizationstates to be used by a polarizer and one or more analyzers, ininvestigating an object to be tested, according to an embodiment of thepresent invention;

FIG. 10 is a flowchart describing a process to determine one or morefunctions of polarization states to be used by a polarizer and one ormore analyzers, in investigating an object to be tested, according to anembodiment of the present invention; and

FIG. 11 shows schematic graphs of block signature values vs. criticaldimension values, according to an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Reference is now made to FIG. 1, which is a schematic diagram of asurface examination apparatus 20, according to an embodiment of thepresent invention. Apparatus 20 acts as a radiation scanner of surfacesof a sample object and test objects, which are typically a referencewafer and production wafers. The apparatus comprises a source 22 ofradiation which generates a beam 30 that is used to irradiate a surface26. Herein, by way of example, surface 26 is assumed to comprise thesurface of a wafer 28, but it will be appreciated that embodiments ofthe present invention may be used for irradiation and/or examination ofsubstantially any surface. Wafer 28 typically has a multiplicity ofsubstantially similar dies 24 formed on its surface. There are amultiplicity of functional features 25, such as memory cells, logiccells, or groups or components thereof, within each die. In thefollowing description, specific dies 24 are distinguished from eachother by adding a letter suffix to the identifying die numeral, forexample die 24A. In addition, specific wafers or types of wafers 28,such as production wafers, or wafers used for calibration, aredistinguished by adding a letter suffix to the identifying wafernumeral, for example production wafer 28P and calibration wafer 28C.Source 22 is typically a laser, although any other suitable source ofradiation, such as a suitable ultra-violet (UV) or deep UV (DUV) source,may be used to generate beam 30. An operator 31 may control apparatus20, or alternatively the apparatus may be configured to operatecompletely automatically.

Embodiments of the present invention may also use a computer simulatedwafer. In the description herein, a simulated wafer is distinguishedfrom a physical wafer by having a letter prefix added to the identifyingwafer numeral of the simulated wafer, for example simulated wafer S28.Computer simulated wafers are typically stored as software instructionsand/or data in a memory 38.

For clarity in the following description, surface 26 is assumed todefine an x-y plane, with the x-axis lying in the plane of the paper,and the y-axis going into the paper. A z-axis is normal to the x-yplane, as shown in FIG. 1. The description also uses, for clarity,positional terms with respect to the figures such as left side and lowersection of the wafer. The axes and the positional terms are arbitrary,and are not to be construed as limiting the scope of the presentinvention.

By way of example, beam 30 is assumed to enter a beam scanning module 34which scans the beam in an x-direction, typically using rotating mirrorsand/or acousto-optic deflectors. Module 34 is part of a set ofprojection optics elements 46. Systems for performing the functions ofmodule 34 are well known in the art. U.S. Pat. Nos. 6,853,475 and6,809,808 to Feldman et al, describe examples of such systems, and areincorporated herein by reference. Module 34 is controlled by a processor36, which also operates other elements of apparatus 20. Alternatively,module 34 is not present in projection optics elements 46, in which caseapparatus 20 may be arranged in an area imaging configuration whereinbeam 30 illuminates an area of surface 26. In this configuration,surface 26 may be scanned by moving the wafer. In both systems, pixelsizes, described in more detail below, are substantially the same. Thosehaving ordinary skill in the art will be able to adapt the descriptionherein, mutatis mutandis, for an area configuration.

Processor 36 is coupled to memory 38, wherein are stored softwareinstructions 39 for operation of the apparatus. Instructions 39 can besupplied to apparatus 20 as a computer software product, in electronicform or on tangible media such as a magnetic storage disc or a compactdisc which are readable by a computer, or by other means known in theart for permanent storage of electronic data.

Projection optics elements 46 include one or more polarizing elements 21which change the polarization of beam 30 under control of processor 36.Beam 30 is typically polarized, in which case elements 21, herein alsotermed polarizer 21, may comprise a half- or a quarter-wave plate, or acombination of these plates, which receives beam 30 and which alters thecharacteristic of the beam exiting polarizer 21. Processor 36 is able torotate the axis of the half- or quarter-wave plate so as to define thecharacteristic of the beam exiting the plate. If beam 30 is notpolarized, elements 21 typically comprise a linear polarizer, which maybe followed by a half- or quarter-wave plate.

Thus, polarizer 21 may convert beam 30 into one of three modes ofpolarized radiation: linearly, circularly, or elliptically polarizedradiation, where the eccentricity e of the elliptically polarizedradiation is in the range 0<e<1. Processor 36 is able to select the modeof polarization for a particular scan, and is also able to defineparameters of the mode. The parameters that the processor defines aredependent on the mode: for the linearly polarized mode, the parameter isthe direction of polarization; for the circularly polarized mode theparameter sets if the polarization is left or right circularly polarizedradiation; and for the elliptically polarized mode the parameter setsthe ellipticity e of the polarized radiation, as well as whether theexiting radiation is left or right elliptically polarized. Thecombination of a mode of polarization, and of the parameter applying tothe mode, is hereinbelow referred to as the polarization state. Forexample, one polarization state is radiation that is linearly polarizedalong a y-axis, wherein the mode is linear and the parameter of the modeis the y-axis direction.

Elements 46 also include other elements, indicated schematically in FIG.1 by a relay lens 42 and an objective lens 44. Projection optics 46function to project and focus radiation from beam 30 onto a spot 49 onsurface 26. Optics 46 also include a beamsplitter 52, which allowstransmission of beam 30 to surface 26.

Typically, spot 49 covers a small region of surface 26, and the size ofthe region typically governs the sizes of rectangular pixels 58 intowhich the surface is assumed to be divided. The size of spot 49, andthus of a pixel, is a function of several factors including thewavelength of source 22 and the numerical aperture of the optics. Thepixel size is typically selected according to the design rule used informing dies 24 on the surface. For example, each pixel 58 may be asquare having a side in a range of 80-160 nm. While pixels 58 takentogether may tile, i.e., completely cover, surface 26, it is not to beconstrued that apparatus 20 necessarily scans the entire surface, sincein some embodiments only selected regions of the surface may be scanned.

Wafer 28 is mounted on a motion stage 29 which is controlled byprocessor 36. Stage 29 rotates and translates the wafer, as well asbringing the wafer into focus, using independent x-, y-, andz-translation stages, as well as one or more rotation stages. Processor36 controls the movement of the motion stages, as well as the scanningperformed by module 34, so that substantially any part of surface 26 maybe irradiated by beam 30. The particular motions generated by module 34and stage 29 are described in more detail below.

Returning radiation from spot 49 is typically of three forms: brightfield radiation, comprising radiation which is substantially specularlyreflected from surface 26, dark field radiation, which is typicallyradiation which is scattered from surface 26 at relatively large anglesto the path of the specular reflected radiation, and gray fieldradiation, which is scattered from surface 26 at angles generallybetween the path of the specular reflected radiation and the dark fieldradiation. Generally, in examining surface 26, all three forms ofreturning radiation may be measured.

In apparatus 20 beam 30 is arranged to be incident substantiallynormally on surface 26, although the apparatus could be set to operateat non-normal angles of incidence. A mirror 54 has a hole at its centerto allow free passage of beam 30, as well as returning specularradiation from spot 49. Specular radiation from spot 49 traversesobjective 44, and is reflected by beamsplitter 52 via a lens 56 to abright field detector 66. In some embodiments a polarizing analyzer 23precedes detector 66. Objective 44, beamsplitter 52, and lens 56 arepart of a set of receiving optics 63.

Mirror 54 reflects gray field radiation from spot 49 to one or more grayfield detectors 50. Detectors 48 measure the incoming dark fieldradiation. In some embodiments polarizing analyzers 27 and 33respectively precede respective gray field detectors and dark fielddetectors. For simplicity, in FIG. 1 a section of one annular gray fielddetector, and two dark field detectors are shown, but any suitablenumber of each type may be used in apparatus 20. Typically, the darkfield and gray field detectors are effectively distributed approximatelysymmetrically about beam 30. By way of example, apparatus 20 is assumedto comprise a total of 6 bright field, gray field, and dark fielddetectors.

Analyzers 23, 27, and 33 are polarizing elements. Except as statedbelow, modes and/or parameters of analyzers 23, 27, and 33, if present,are set by processor 36, to correspond with the mode and parameter ofpolarizer 21.

Returning radiation from spot 49 impinges on each of detectors, 66, 50,and 48, and the detectors in turn generate respective signal levels foreach pixel 58 according to their received radiation intensity. Thespecific signal levels for each detector are characteristic of thefunctional and/or structural features of the pixel being irradiated, aswell as of the intensity and direction of polarization of beam 30. Thus,a matrix formed from up to 6 different detector levels, or receivedradiation intensity levels, may be associated with each pixel 58,element values of the matrix for a particular pixel being dependent onthe functional/structural features of the pixel. For example, a pixelwhich comprises a conducting material line, such as is commonly used insemiconductor wafers, typically has a significantly different matrixcompared to the matrix associated with an insulator.

As is described above, apparatus 20 may monitor bright field, grayfield, and dark field returning radiation. Alternatively, apparatus 20may not monitor all of these returning radiations. Except whereotherwise stated, for simplicity the following description assumes thatonly signals from returning bright field radiation are used, so thatonly detector 66 is operative. Those having ordinary skill in the artwill be able to adapt the description, mutatis mutandis, for embodimentswherein signals from bright, gray, and/or dark fields are used.

Typically, processor 36 examines surface 26 as quickly as possible. Theprocessor typically performs the examination by irradiating consecutivepixels of the surface, each pixel being examined by scanning beam 30 inthe x-direction while translating surface 26 in the y-direction. Toreduce the time spent on examination, processor 36 may set the rate ofscanning of beam 30 and the rate of translation of surface 26 as high aspossible, while providing an acceptable signal to noise level atdetector 66, and while maintaining an acceptable number of pixels perspor 49. Typically, rates of mechanical scanning across surface 26 arein the range 80-260 mm/s.

By way of example, the description herein assumes that apparatus 20 usesone beam 30 which irradiates a single spot 49 of surface 26. Methods areknown in the art to irradiate multiple spots of surface 26 substantiallysimultaneously, for example, by multiplexing beam 30 into a multiplicityof beams, and/or by using multiple radiation sources. Those havingordinary skill in the art will be able to adapt the description herein,mutatis mutandis, for systems irradiating multiple spots and derivingmeasurements of respective regions of an irradiated surfacesubstantially simultaneously.

FIG. 2 is a schematic diagram of surface 26, according to an embodimentof the present invention. The surface is assumed to be divided intosubstantially congruent dies 80, and each of the dies is typicallyeventually used as part of an electronic device such as a memory. Usingan exemplary pixel dimension of 100 nm, within the range exemplifiedabove, for a die that is a square having side 1 mm, each die comprises10,000×10,000 pixels. By way of example, dies and the pixels comprisingthe dies are assumed to be arranged as Cartesian arrays, so that anyspecific die or any specific pixel may be uniquely identified by anx-coordinate and a y-coordinate.

In embodiments of the present invention, relatively large numbers ofpixels are grouped into equal-size blocks 82. For a grouping of 50×50pixels forming blocks 82, each die 80 of side 1 mm is formed of 200×200blocks. In one embodiment block 82 is a group of 32×32 pixels. However,it will be understood that each block 82 may comprise any convenientnumber of pixels. It will also be understood that although block 82 istypically rectangular, there is no requirement that this condition besatisfied, so that the block may be formed as any convenient closedfigure of contiguous pixels.

A block signature is determined for any specific block 82. Each blocksignature is formed from a histogram of measured values of the pixels ofthe block. The histogram may be one-dimensional (1D), for exampleplotting frequencies of intensities of bright field returning radiationfrom the pixels of the block. Alternatively, the histogram may betwo-dimensional (2D), for example plotting frequencies of ordered pairs(intensity, intensity spread of neighboring pixels) of bright fieldreturning radiation from the pixels of the block.

Further alternatively, in general the histogram for a block signaturemay be n-dimensional (nD), where n is a natural number, plottingfrequencies of ordered n-tuples (x₁, x₂, x₃, . . . x_(n)) where x₁ . . .are parameters of a pixel measurement. An example of a four-dimensionalhistogram that may be used for a block signature plots frequencies ofthe 4-tuple (I_(b), S_(b), V_(b), I_(g)), where I_(b) is the returningbright field radiation intensity from a specific pixel in the block,S_(b) is the spread of returning bright field radiation intensities fornearest neighbors of the specific pixel, V_(b) is the variation ofS_(b), and I_(g) is the returning gray field intensity from the specificpixel.

The block signature is a number representative of the values of thehistogram. For example, for a 1D histogram a percentage of thedistribution, an arithmetic mean, or a median of the histogram valuesmay be calculated, and the percentage, the mean or the median used asthe block signature.

In an embodiment of the present invention, a block signature isdetermined for a specific polarization state of the radiation incidenton surface 26. As is described in more detail below, the specificpolarization state is typically determined by constructing blocksignatures for different polarization states, and determining the statethat gives an optimal signature for a parameter or parameters beinginvestigated, initially by examination of a calibration wafer.

FIG. 3 is a flowchart 100 describing a process to determine apolarization state to be used in investigating a production wafer, andFIG. 4 shows tables formed by processor 36 in performing the process,according to embodiments of the present invention. The process describedby flowchart 100 is assumed to be performed by processor 36 usinginstructions 39, and is to find a state of polarized radiation withwhich any object to be tested is scanned. Hereinbelow, by way ofexample, objects to be tested are assumed to be production wafers 28P.

There are two sections to the process described by flowchart 100: afirst, results generation section 101, and a second, analysis section105.

In section 101, in a preparation step 102, a calibration object isprepared. The calibration object may comprise a calibration wafer, aproduction wafer having a calibration region in the production wafer, orany other suitable object that provides a known variation of theparameter or parameters that are to be measured. Hereinbelow, by way ofexample the calibration object is assumed to comprise a calibrationwafer 28C which is physically fabricated. In some embodiments, a focusexposure matrix (FEM) wafer is used as calibration wafer 28C. As isknown in the art, an FEM wafer comprises a wafer in which a reticleexposes a mask with multiple combinations of focus and exposure settingsonto the wafer that has been coated with a photoresist. The FEM wafer ischaracterized, typically with a CD-SEM (critical dimension scanningelectron microscope) to determine resist profiles and line widths, andthe corresponding focus and exposure settings, that most closely match adesired profile and line width. Alternatively, parameters of thecalibration object are simulated.

In the description of flowchart 100, preparation step 102 is assumed tocomprise physical fabrication of calibration wafer 28C, and those havingordinary skill in the art will be able to adapt the description, mutatismutandis, to the case where parameters of the calibration wafer aresimulated.

In the following description, except where otherwise stated, thecalibration wafer is assumed to be formed to have a known variation ofthe parameter or parameters that are to be measured, also herein termedthe parameter- or parameters-under-investigation, across productionwafers 28P. For the calibration wafer, typical parameters formed with aknown variation include, but are not limited to, line width, lineheight, and line spacing. Those having ordinary skill will be aware ofother parameters, such as line edge roughness, surface roughness,sidewall angle, composition, or amount of doping, that may vary across awafer, and all these parameters are included in the scope of the presentinvention.

By way of example, in the description of flowchart 100 herewith, oneparameter is assumed to be measured, and the parameter is assumed to bea line width. (Flowchart 300, below, describes a process wherein morethan one parameter is measured.) An acceptable range for the line widthsof production wafers 28P is typically approximately +/−5% from a nominalvalue. Wafer 28C is fabricated so that the line widths vary within thisrange, and so that the variation is substantially linear along one ofthe axes of the wafer. The axis of variation is assumed to correspond tothe y-axis of the wafer.

In a reference step 103 processor 36 generates a reference calibrationtable 150 (FIG. 4) of the line width as it varies along the y-axis ofthe wafer, the table comprising one-to-one correspondences ofy-coordinates of the wafer and line widths at the y-coordinate. By wayof example, the line width of the conductors is assumed to be determinedby processor 36 scanning different regions of wafer 28C using a scanningelectron microscope (SEM). The processor then calculates the line widthsfor different values of y from the scanned results, to produce table150, and the processor stores values of the table in memory 38 for uselater in the process described by flowchart 100. Although the varyingparameter is assumed herein to be measured using an SEM, processor 36may use any convenient system known in the art, for example OpticalCritical Dimension (OCD) metrology or a scanning X-ray microscope (SXM),to generate reference calibration table 150.

In a set polarization state step 104, processor 36 adjusts elements ofapparatus 20 so that the y-axis of calibration wafer 28C corresponds tothe y-axis of the apparatus. By way of example, polarizer 21 ishereinbelow assumed to be set to generate linear polarization, andprocessor 36 also sets the direction of the linear polarization. Ifpresent, analyzers 23, 27, and/or 33 are also set to transmit linearpolarized radiation, and the direction of transmission is set tocorrespond to the direction set for polarizer 21. Alternatively, thepolarization state set in step 104 may comprise circular or ellipticalpolarization.

In a scan step 106, processor 36 scans calibration wafer 28C. Theprocessor stores the intensity measured at bright field detector 66, foreach pixel of the calibration wafer in memory 38, in the form of a pixeltable 152A of pixel identifiers (IDs), determined from the x-coordinatey-coordinate of each pixel, and intensities of the corresponding pixels.Table 152A thus comprises one-to-one correspondences of unique pixel IDsP1, P2, . . . and intensities IP1A, IP2A, . . . of the identified pixel.In other embodiments, the intensities measured at gray field detectors50 and/or dark field detectors 48 are also stored and incorporated intotable 152A.

As indicated by a line 108, processor 36 repeats steps 104 and 106 fordifferent polarization states, the number of different states typicallybeing controlled by operator 31. The polarization state is changed in achange polarization step 109. In the case of linear polarization, thestates include at least two different orthogonal polarizations. Thus, instep 106, there are a respective number of tables 152A, 152B, . . .formed, the number corresponding to the number of different polarizationstates implemented in steps 104 and 106, each of the tables having thesame one-to-one correspondence format as table 152A. Tables 152A, 152B,. . . are herein also referred to generically as tables 152.

Results generation section 101 completes when a final scan step 106 hasbeen performed. The following steps of flowchart 100 comprise resultsanalysis section 105.

In a block formation step 110, processor 36 selects groups of contiguouspixels that are to be considered as respective blocks. The selection maybe performed on a completely automatic basis, for example by operator 31defining a size of a rectangular block before the process of flowchartis begun. Alternatively, the operator may, for example on an iterativebasis, set different sizes for the group of pixels to be considered as ablock. Although blocks are typically selected to be rectangular, this isnot a requirement, so that blocks having non-rectangular shapes may alsobe defined.

In a block signature step 112, for each polarization state scanned instep 106, processor 36 calculates the block signature BS1, BS2, . . . ofeach block of pixels, using the intensities stored in tables 152. Inaddition, processor 36 assigns a unique block ID, B1, B2, . . . derivedfrom a block's x- and y-coordinates, to each block. The processor usestables 152 to find intensities of pixels within each of the blocks. Byway of example, a block signature of a given block is calculated as thearithmetic mean of the intensities of the pixels in the given block. Ingeneral, a given block signature BSn (n a natural number) is a functionof the intensities of the pixels in block Bn. Processor 36 generatesfrom tables 152A, 152B, . . . respective tables 154A, 154B, . . . , eachtable 154A, 154B, . . . having a one-to-one correspondence of block IDsand block signatures, BSnA, BSnB, . . . . Tables 154A, 154B, . . . arealso referred to generically herein as tables 154.

In a first analysis step 116, processor 36 groups the blocks accordingto the axis of variation of the parameter being measured, in this caseaccording to the y-coordinates of the blocks. The processor thencalculates the mean block signature for each group of blocks having thesame y-coordinates, and forms as respective tables 156A, 156B,one-to-one correspondences of y-coordinates and mean block signaturesMBS1A, MS2A, . . . MBS1B, MBS2B, . . . at the y-coordinates. Tables156A, 156B, . . . are also referred to generically herein as tables 156.

In some embodiments of the present invention, processor 36 reiteratessteps 110 to 116 using a different definition for a block, the differentdefinition typically comprising a change in rectangular dimensions ofthe block. Alternatively or additionally, the different definition of ablock may comprise a change in shape of the block. The reiteration isillustrated in flowchart 100 by a broken line 120, and a change blockdefinition step 122.

In a second analysis step 118, processor 36 correlates the mean blocksignatures of each of tables 156 with the line widths of thecorresponding y-coordinates, as stored in table 150. The processor usesthe correlation to determine which table of mean block signatures givesthe highest variation of block signatures for the variation of linewidths. In other words, the processor determines a maximum sensitivityof the block signatures for the changes in line width. Typically, forevery table 156A, 156B, . . . processor 36 performs a linear regressionanalysis of the mean block signatures with theparameter-under-investigation of table 150, and determines from theregression analysis a variation of block signatures corresponding to thevariation of line widths, as well as a correlation coefficient, for eachtable 156A, 156B, . . . .

In a selection step 124, the processor determines which table 156A,156B, . . . herein termed an optimal table 156O, gives the maximumsensitivity of the block signatures. The processor stores thepolarization state used to generate optimal table 156O in memory 38. Theprocessor also stores the block definition used to generate optimaltable 156O in memory 38.

In a final step 126, the processor scans production wafers 28P with thepolarization state determined in selection step 124, substantially asdescribed for steps 104 and 108 above. From the scan results processor36 determines block signatures of blocks in wafers 28P, typically usingthe block definition of step 124 and the method for determining blocksignatures used in step 112. In some embodiments the block definitionused for production wafers 28P is altered, typically so as to improve asignal to noise ratio of the block signatures. Operator 31 uses theblock signatures determined in step 126 to estimate variations in theparameter-under-investigation across the production wafers 28P.

Flowchart 100 then ends.

In some embodiments, as well as (as described in step 102 of flowchart100) forming the parameter of the calibration wafer to have a knownvariation, a wafer may be formed so that theparameter-under-investigation does not vary over the wafer. In thiscase, in addition to finding the polarization state giving the maximumsensitivity for the block signature variations using the wafer with theknown variation, the polarization state and block definition found forthe varying parameter may be tuned to give block signatures that do notvary using the wafer with the non-varying parameter. Those havingordinary skill in the art will be able to adapt the description offlowchart 100 above, mutatis mutandis, to determine an optimalpolarization state giving block signatures which do not vary, by, forexample, in steps 110 and 122 finding a block definition, in step 118finding correlation coefficients, and in step 124 finding thepolarization state giving the highest correlation coefficient.

Wafer measurements using systems such as a CD-SEM are extremelytime-consuming, and consequently costly, although the measurements havea high characteristic resolution. Measuring a small region of one wafertypically takes of the order of 10 minutes. If a complete wafer ismeasured with a CD-SEM, the time is many hours, if not days. Theinventors have found that results from the process of flowchart 100 maybe as good as those produced by wafer measurement system such as aCD-SEM, although apparatus 20 typically has a resolution one or moreorders of magnitude less than the characteristic resolution of systemssuch as the CD-SEM. Furthermore, the process of flowchart 100 issignificantly less time-consuming than that of the CD-SEM system, andthis is particularly true if, as is required for production wafers, thewhole wafer is to be measured. The inventors have found that a completeproduction wafer may be satisfactorily scanned, giving block signatureswith good signal to noise ratios, typically in less than one hour.Moreover, the process of flowchart 100 may be combined with otheroptical scanning measurements performed by apparatus 20, such asdie-to-die comparisons, simultaneously. Thus, the results of a singleoptical scan of a production wafer, typically taking less than one hour,may be used to generate block signatures of all blocks of the wafer, asdescribed in step 126, as well as to perform die-to-die comparisons. Theoverall savings in time in measuring production wafers are consequentlyconsiderable.

FIG. 5 is a flowchart 300 describing a process to determine one or morefunctions of polarization states to be used in investigating aproduction wafer, and FIG. 6 shows tables formed by processor 36 inperforming the process, according to embodiments of the presentinvention. Apart from the differences described below, the processdescribed by flowchart 300 is generally similar to the process offlowchart 100 (FIGS. 3 and 4) and operations performed in stepsindicated by the same reference numerals in both flowchart 100 andflowchart 300 are generally similar.

FIG. 7 shows two graphs 400 and 402 of reflectivity vs. line width, fortwo modes of linearly polarized radiation, according to an embodiment ofthe present invention. Graphs 400 and 402 are prepared by simulation.Both graphs plot reflectivity vs. line width, the line widths beingwidths of a set of parallel lines where the line plus space is constant,giving a constant pitch. Graph 400 shows the reflectivity for a firstmode of linearly polarized radiation that is parallel to the set ofparallel lines. Graph 402 show the reflectivity for a second mode oflinearly polarized radiation that is perpendicular to the set ofparallel lines. Simulated results such as those illustrated by graphs400 and 402 are used in the process of flowchart 300.

Returning to FIG. 5, the process described by flowchart 300 is assumedto be performed by processor 36 using instructions 39. The process findsmultiple polarization states with which production wafers 28P are to bescanned, and determines expressions having the multiple polarizationstates that are to be used in generating block signatures of the wafers.

In preparation step 102 a simulated calibration wafer S28C is generated.By way of example, wafer S28C is assumed to have a linear variationalong the x-axis of a line width, and a linear variation along they-axis of a line height.

In a reference step 301, which is generally similar to reference step103, the line width variation is set at +/−11% from a nominal linewidth. The line height variation is set at +/−10% from a nominal lineheight. Processor 36 produces two reference calibration tables 350X and350Y, table 350X having a correspondence between the x-coordinate of thewafer and the line width, table 350Y having a correspondence between they-coordinate of the wafer and the line height (FIG. 6).

A simulated scan step 302 replaces steps 104, 108, and 109 of flowchart100. In step 302 processor 36 performs multiple simulated scans of waferS28C, each scan using a different polarization state. Hereinbelow themultiple polarization states are assumed, by way of example, to consistof a first scan of linearly polarized radiation with the direction ofpolarization being parallel to the set of lines in wafer S28C, and asecond scan of linearly polarized radiation with the direction ofpolarization being perpendicular to the set of lines in the simulatedwafer. The two scans generate tables 352 and 353, respectively forparallel and perpendicular radiation. Each table is generally similar toone of tables 152, having a correspondence between pixel identities P1,P2, . . . and intensity of radiation returning from the pixel. Theintensities for table 352 are assumed to be A1, A2, A3, . . . ; theintensities for table 353 are assumed to be E1, E2, E3, . . . .

However, there is no requirement that only two states of polarizedradiation are used in step 302, or that the states used have any specialrelation with the set of lines of the simulated wafer, and in generalmore than two states may be used. Thus, for linearly polarized radiationthe directions of polarization may be selected to be substantially anyof two or more directions relative to the set of lines being examined,and for elliptically polarized radiation the ellipticity may be set tobe any values between 0 and 1.

Step 110 is generally as described for flowchart 100.

A step 304 replaces step 112. In step 304 signatures of blocks definedin step 110 are calculated as composite signatures formed from theintensity values in tables 352 and 353. For example, a block compositesignature for a block comprising pixels P1, P2, P3 uses intensities A1,E1, A2, E2, A3, E3. In general a composite block signature CBSn is afunction of the intensities from tables 352 and 353 for the pixels inblock Bn.

In step 304 processor 36 forms a set of tables 354, typically under thedirection of operator 31, each table having a one-to-one correspondenceof a block identity and a composite signature. Tables 354 are generallysimilar to tables 154. By way of example, three tables 354A, 354B, and354C are shown in FIG. 7, but in general the number of tables in the setof tables may be any convenient integer greater than or equal to two.

For any particular table 354, the expression used to form the compositeblock signatures of the table is typically a linear expression of thepixel intensities of tables 352 and 353. However, other expressions,such as a quadratic or other non-linear expression of the pixelintensities, may be used to form the composite block signatures.

Processor 36 implements steps 116, 122, and 118 generally as describedabove for flow chart 100. However, the correlation in step 118 isperformed by correlating tables 354 with each reference table generatedin step 102.

In addition, the tables formed in step 116 are formed by grouping theblocks of tables 354 according to the different axes used in referencewafer S28C. Thus in step 116 the blocks are grouped according tox-coordinates and according to y-coordinates, forming tables 356. Eachtable 354 is formed into one table having a one-to-one correspondence ofx-coordinates and mean block signatures at the x-coordinates and asecond table having a one-to-one correspondence of y-coordinates andmean block signatures at the y-coordinates. Thus table 354A generates anx-coordinate table 356XA and a y-coordinate table 356YA; table 354Bgenerates an x-coordinate table 356XB and a y-coordinate table 356YB;and table 354C generates an x-coordinate table 356XC and a y-coordinatetable 356YC.

A selection step 306 is generally similar to step 124 described abovefor flowchart 100. However, in flowchart 300 for step 306 processor 36determines two optimal tables. A first optimal table 356XO, selectedfrom tables 356XA, 356XB, . . . has a maximum sensitivity for theparameter of table 350X; a second optimal table 356YO, selected fromtables 356YA, 356YB, . . . has a maximum sensitivity for the parameterof table 350Y. The processor stores the polarization states used togenerate optimal tables 356XO and 356YO in memory 38, and also storesthe respective expressions, used in step 304, that form the optimaltables. In addition, the processor stores the block definition used togenerate the optimal tables in the memory.

In some embodiments, prior to testing production wafers, processor 36optimizes the polarization states, the expressions, and the blockdefinition stored in step 124 by testing on a physical calibration suchas the FEM wafer described above with reference to flowchart 100. Theoptimization is performed in an optimization step 308, and valuesdetermined from the optimization step are stored in memory 38. Theoptional character of step 308 is indicated by broken lines in FIG. 5.

Processor 36 implements final step 126, generally as described forflowchart 100, on production wafers 28P using the values stored inmemory 38. Since more than one polarization state is stored, wafers 28Pare scanned using all polarization states that have been determined instep 306. The multiple scans at the respective different polarizationstates are typically performed sequentially. In some embodiments, themultiple scans may be performed simultaneously, for example by usingmultiple simultaneous beams to irradiate surface 26.

From the results generated using the multiple scans, processor 36 usesthe expressions stored in the memory to determine composite blocksignatures of the wafer, as described in step 304. Operator 31 uses thecomposite block signatures to estimate variations in theparameters-under-investigation across the production wafers 28P.

Flowchart 300 then ends.

FIG. 8 shows exemplary results obtained using the process of flowchart300, according to an embodiment of the present invention.

An optimal table 356XO was derived by using, in step 304, a first linearequation (1)

In=3.5 En−An   (1)

An optimal table 356Y0 was formed by using in step 304 a second linearequation (2):

In=En+0.6 An   (2)

In equations (1) and (2), n is a pixel identifier and In is a compositeintensity generated by the pixel. The composite block signatures of step304 are formed by summing values of In over all the pixels of a block.

Grid arrays 502 and 504 plot line height on a vertical axis and linewidth on a horizontal axis, the values corresponding to those ofsimulated wafer calibration wafer S28C (described above with referenceto step 103 of flowchart 300).

Grid array 502 shows approximate values of composite block signatures,determined using optimal table 356XO, i.e., composite block signaturesbased on equation (1). Examination of the block signatures of grid array502 shows that the composite block signatures have high sensitivity forchanges of line width, and are substantially invariant with changes ofline height. Thus equation (1) generates good composite pixelintensities and corresponding composite block signatures for examiningvarying line widths even in the presence of varying line heights.

Grid array 504 shows approximate values of composite block signatures,determined using optimal table 356YO, i.e., composite block signaturesbased on equation (2). Examination of the block signatures of grid array504 shows that the composite block signatures have high sensitivity forchanges of line height, and are substantially invariant with changes ofline width. Thus equation (2) generates good composite pixel intensitiesand corresponding composite block signatures for examining varying lineheights even in the presence of varying line widths.

Embodiments of the present invention replace the time-consumingmeasurements of systems such as SEMs with methods that are orders ofmagnitude faster than those of SEMs or comparable systems, and theinventors have found that the results from the two types of systems arecomparable. Because of their time consuming properties, systems such asSEMs or OCD measuring devices are typically only used on portions of awafer. The faster operation of embodiments of the present inventionmeans that a whole wafer can be measured in a relatively short time.

Embodiments of the present invention may be used in a wide range ofapplications wherein measurements of periodic structures are required.Some of the applications are itemized in pages 7 and 8 of U.S.Provisional Patent Application 60/950,077, incorporated herein.

Because of the relatively short time used by embodiments of the presentinvention, they may be applied in both feed-forward and feed-backsystems. For example, examination of a present wafer may produce blocksignatures showing that line widths are close to a permissible rangeboundary, in which case feedback, such as processor 36 increasing ordecreasing the intensity of radiation on a succeeding wafer, may beapplied.

Returning to FIG. 1, in some embodiments of the present invention, atleast one of analyzers 23, 27 and 33 is present, and the polarizationstate of at least some of the analyzers is set to be different from thepolarization state of polarizer 21. Some examples of differentpolarization states are given in Table I below. In the table, thereference used for linear and elliptical polarizations is assumed, byway of example, to be the y-axis of the wafer, although any otherconvenient reference may be used. The polarization state for thepolarizer and the polarization state for the one or more analyzers, arerespectively also referred to herein as the polarizer setting and theanalyzer settings.

TABLE I Polarization state for Polarizer Polarization state for Analyzeror Polarizer Setting AnalyzersAnalyzer Settings Mode Parameter ModeParameter 1 Linear  0° to reference Linear  30° to reference 2 Linear20° to reference Linear 110° to reference 3 Circular Left hand CircularRight hand 4 Elliptical Right hand, 0.5 Elliptical Left hand, 0.5eccentricity, eccentricity, major major axis at 45° to axis at 45° toreference reference 5 Elliptical Right hand, 0.3 Elliptical Right hand,0.8 eccentricity, major eccentricity, major axis at 135° to axis at 45°to reference reference 6 Linear  0° to reference Circular Left hand 7Circular Right hand Elliptical Right hand, 0.8 eccentricity, major axisat 45° to reference 8 Elliptical Right hand, 0.5 Linear   0° toreference eccentricity, major axis at 0° to reference 9 Linear +45° toreference Linear −45° to reference 10 Linear −45° to reference Linear+45° to reference

FIG. 9 is a flowchart 600 describing a process to determine polarizationstates to be used by polarizer 21 and analyzers 23, 27, and/or 33, ininvestigating an object to be tested, according to an embodiment of thepresent invention. Apart from the differences described below, the stepsof the process of flowchart 600 are generally similar to those of theprocess of flowchart 100, and steps indicated by the same referencenumerals in both flowcharts are generally similar in implementation. Asfor flowchart 100, objects to be tested are assumed to be productionwafers 28P.

For clarity and simplicity, in the description of flowchart 600, and aflowchart 700 (FIG. 10) below, it is assumed that the polarization statefor only one analyzer, herein assumed to be one gray field analyzer 27,i.e., the analyzer setting, is determined. In this case the polarizationstates of the polarizer and the analyzer are different. Those havingordinary skill in the art will be able to adapt the descriptions forflowchart 600 and 700, mutatis mutandis, to accommodate the changesrequired for the case of more than one analyzer, and/or for cases wheresome detectors have an analyzer, and some do not. In these cases, allthe analyzers may have the same polarization state, different from thatof the polarizer. Alternatively, some of the analyzers may havedifferent polarization states from each other, and some, but not all,may have the same polarization state as the polarizer. Thus, dependingon the configuration of apparatus 20 all the returning radiation istransmitted through one or more analyzers to respective detectors, oronly a part of the returning radiation is transmitted through one ormore analyzers to respective detectors, and the remainder of thereturning radiation is detected without being transmitted through ananalyzer. In the specification and in the claims, the at least partiallyanalyzed returning radiation is also herein termed the processedreturning radiation.

The process of flowchart 600 uses tables similar to those of FIG. 4. Theprocess determines the polarization state for the polarizer, and thepolarization state for the analyzer that gives maximum sensitivity of ablock signature for the variation of a parameter-under-investigation.

In flowchart 600 steps 102 and 103 are substantially as described abovefor flowchart 100.

A set polarization states step 602 replaces step 104. In step 602,processor 36 adjusts elements of apparatus 20 so that the y-axis ofcalibration wafer 28C corresponds to the y-axis of the apparatus. Thepolarization states for the polarizer and the analyzer are also set. Byway of example, polarizer 21 is hereinbelow assumed to be set togenerate linear polarization, and processor 36 also sets the directionof the linear polarization, assumed to be parallel to the y-axis. Theprocessor also sets the polarization state for analyzer 27, hereininitially assumed to be orthogonal to the direction of the polarizer.However, any other different polarization states, such as thoseexemplified in Table I, may be used in step 602.

Step 106 is substantially as described for flowchart 100.

Step 109 is generally as described for flowchart 100, except that thepolarizer and/or the analyzer polarization states may be changed. In oneembodiment the change maintains the orthogonality between the twopolarization states.

Steps 110, 112, 116, 122, 118 and 124 are generally as described abovefor flowchart 100, except that in step 124, the polarization states forthe polarizer and for the analyzer are stored.

In a final step 604, which replaces step 126, production wafers 28P arescanned with polarizer 21 and analyzer 27 set to the polarization statesstored in step 124.

Flowchart 600 then ends.

FIG. 10 shows flowchart 700, which describes a process to determine oneor more functions of polarization states to be used by polarizer 21 andanalyzer 27, in investigating an object to be tested, according to anembodiment of the present invention. Apart from the differencesdescribed below, the process described by flowchart 700 is generallysimilar to the process of flowchart 300 (FIGS. 5 and 6) and operationsperformed in steps indicated by the same reference numerals in bothflowchart 300 and flowchart 700 are generally similar. The process offlowchart 700 finds multiple polarization states with which objectsunder test, herein assumed to be production wafers 28P, are to bescanned, and determines expressions having the multiple polarizationstates that are to be used in generating block signatures of the wafers.The process determines respective expressions, and the polarizationstates of the polarizer and of the analyzer for the expressions,providing maximum sensitivities for parameters-under-investigation.

Steps 102 and 301 are substantially as described above for flowchart300, wherein a simulated calibration wafer is generated. As forflowchart 300, the wafer is assumed to be wafer 28C, with a linearvariation along the x-axis of a line width, and a linear variation alongthe y-axis of a line height, and processor 36 produces two referencecalibration tables 350X and 350Y (FIG. 6).

A simulated scan step 702 replaces step 302 of flowchart 300. In step702 processor 36 performs multiple simulated scans of wafer S28C, eachscan having a different set of polarization states for polarizer 21 andanalyzer 27. In each set of polarizations the polarization of polarizer21 is different from that of analyzer 27. For example, one scan may havethe polarization state for the polarizer set as linear, at 10° to areference axis, and the analyzer as linear set at 100° to the referenceaxis, so that the polarization states for this scan are linear andorthogonal. Subsequent scans may maintain the orthogonality, whilechanging the angle to the reference is changed. Alternatively oradditionally, other polarization states, for example those similar to oras given in Table I, may be applied in step 702.

Steps 110, 304, 116, 122, and 118 are substantially as described abovefor flowchart 300.

Steps 704, 706, and 708 are respectively generally similar to steps 306,308, and 126 of flowchart 300, except that the different polarizationstates for polarizer 21 and analyzer 27 are stored. After an optionaloptimization, the different polarization states are used to scan testobjects.

Flowchart 700 then ends.

FIG. 11 shows, in a non-limiting manner, schematic graphs of blocksignature values vs. critical dimension values, according to anembodiment of the present invention. The graphs of FIG. 11 are derivedfrom results generated by the inventors from simulated scans of a wafer,using a process corresponding to flowchart 600 (FIG. 9). The wafer'ssurface is simulated to have a varying critical dimension (CD) along itsx-axis. The surface comprises resist lines parallel to the y-axis,placed on top of a bottom anti-reflection coating (BARC) stack, and thestructure is configured to have a period that changes along the x-axis.The block signature values assume blocks of 50 pixels, and the signatureis the average of returning radiation measured at a bright fielddetector.

Graph “A” shows changes of block signature values for a first state ofpolarizer and analyzer setting. For Graph A the polarizer settingprovides linearly polarized radiation at 45° to the y-axis to irradiatethe wafer, and the analyzer setting assumes that the bright fielddetector measures bright field radiation from the wafer, with ananalyzer before the detector set to linearly polarize the receivedradiation in a direction orthogonal to the irradiating radiation.

Graph “B” shows changes of block signature values for a second state ofpolarizer and analyzer setting. The polarizer and analyzer settings aresimilar to those for graph A, both providing linear polarization, withthe analyzer orthogonal to the polarizer. However, in the case of graphB the polarizer is set to irradiate the wafer with linearly polarizedradiation parallel to the y-axis.

FIG. 11 shows the changes in block signatures for the two sets ofsettings, as sensitivity A and sensitivity B. From inspection of thegraphs, sensitivity A corresponds to the maximum sensitivity, so thatthe settings for graph A, described above, are typically used to inspectphysical surfaces corresponding to those in the simulated exampledescribed herein.

It will be appreciated that the embodiments described above are cited byway of example, and that the present invention is not limited to whathas been particularly shown and described hereinabove. Rather, the scopeof the present invention includes both combinations and subcombinationsof the various features described hereinabove, as well as variations andmodifications thereof which would occur to persons skilled in the artupon reading the foregoing description and which are not disclosed inthe prior art.

1. A method for characterizing a surface of a sample object, the methodcomprising: dividing the surface into pixels which are characterized bya parameter variation; defining blocks of the surface as respectivegroups of the pixels; irradiating the pixels in multiple scans over thesurface with radiation having different, respective first polarizationstates; receiving returning radiation from the pixels in response toeach of the scans; analyzing at least part of the returning radiation ofthe scans using respective second polarization states to generateprocessed returning radiation from the returning radiation; detectingthe processed returning radiation; for each scan, constructingrespective block signatures of the blocks in response to the processedreturning radiation from the group of pixels in each block; for eachscan, determining a block signature variation using the respective blocksignatures of the blocks; and in response to the block signaturevariation, selecting one of the first polarization states and at leastone of the second polarization states for use in subsequent examinationof a test object.
 2. The method according to claim 1, wherein thereturning radiation comprises a first portion and a second portion, andwherein analyzing the at least part of the returning radiation comprisesanalyzing only the first portion.
 3. The method according to claim 2,wherein the second polarization states correspond to the firstpolarization states.
 4. The method according to claim 2, wherein thesecond polarization states are different from the first polarizationstates.
 5. The method according to claim 4, wherein the firstpolarization states comprise first linear polarizations, and wherein thesecond polarization states comprise second linear polarizationsorthogonal to the first linear polarizations.
 6. The method according toclaim 2, wherein a first group of the second polarization statescorresponds to the first polarization states and a second group of thesecond polarization states is different from the first polarizationstates.
 7. The method according to claim 1, wherein the at least part ofthe returning radiation comprises all the returning radiation.
 8. Themethod according to claim 7, wherein the second polarization statescorrespond to the first polarization states.
 9. The method according toclaim 7, wherein the second polarization states are different from thefirst polarization states.
 10. The method according to claim 9, whereinthe first polarization states comprise first linear polarizations, andwherein the second polarization states comprise second linearpolarizations orthogonal to the first linear polarizations.
 11. Themethod according to claim 7, wherein a first group of the secondpolarization states corresponds to the first polarization states and asecond group of the second polarization states is different from thefirst polarization states.
 12. The method according to claim 1, whereinconstructing the respective block signatures comprises defining a blocksignature function of respective intensities of the returning radiationfrom the pixels in a given block, and evaluating the block signaturefunction for the blocks.
 13. A method for characterizing a surface of asample object, the method comprising: dividing the surface into pixelswhich are characterized by a parameter variation; defining blocks of thesurface as respective groups of the pixels; irradiating the pixels inmultiple scans over the surface with radiation having different,respective first polarization states; receiving returning radiation fromthe pixels in response to each of the scans; analyzing at least part ofthe returning radiation using respective second polarization states togenerate processed returning radiation from the returning radiation;detecting the processed returning radiation; constructing respectivecomposite block signatures of the blocks in response to the processedreturning radiation from the group of pixels in each block; determininga composite block signature variation using the block signatures of theblocks; and in response to the composite block signature variation,selecting two or more of the first polarization states and at least oneof the second polarization states for use in subsequent examination of atest object.
 14. The method according to claim 13, wherein constructingthe respective composite block signatures comprises defining a compositeblock signature function of intensities of the returning radiation fromat least two of the multiple scans and from the pixels in a given block,and evaluating the composite block signature function for the blocks.15. Apparatus for characterizing a surface of a sample object,comprising: a processor which is configured to: divide the surface intopixels which are characterized by a parameter variation, and defineblocks of the surface as respective groups of the pixels; and aradiation scanner which is configured to: irradiate the pixels inmultiple scans over the surface with radiation having different,respective first polarization states, receive returning radiation fromthe pixels in response to each of the scans, analyze at least part ofthe returning radiation of the scans using respective secondpolarization states to generate processed returning radiation from thereturning radiation, and detect the processed returning radiation, andwherein the processor is further configured to: for each scan, constructrespective block signatures of the blocks in response to the processedreturning radiation from the group of pixels in each block, for eachscan, determine a block signature variation using the block signaturesof the blocks, and in response to the block signature variation, selectone of the first polarization states and at least one of the secondpolarization states for use in subsequent examination of a test object.16. Apparatus for characterizing a surface of a sample object,comprising: a processor which is configured to: divide the surface intopixels which are characterized by a parameter variation, and defineblocks of the surface as respective groups of the pixels; and aradiation scanner which is configured to: irradiate the pixels inmultiple scans over the surface with radiation having different,respective first polarization states, receive returning radiation fromthe pixels in response to each of the scans, analyze at least part ofthe returning radiation using respective second polarization states togenerate processed returning radiation from the returning radiation, anddetect the processed returning radiation; wherein the processor isfurther configured to: construct respective composite block signaturesof the blocks in response to the returning radiation from the group ofpixels in each block, determine a composite block signature variationusing the block signatures of the blocks, and in response to thecomposite block signature variation, select two or more of the firstpolarization states and at least one of the second polarization statesfor use in subsequent examination of a test object.
 17. A method forcharacterizing a surface of a sample object, the method comprising:simulating the surface as a simulated surface, and defining simulatedcharacteristics of the simulated surface; dividing the simulated surfaceinto pixels which are characterized by a parameter variation; definingblocks of the simulated surface as respective groups of the pixels;simulating irradiation of the pixels in multiple scans over thesimulated surface with radiation having different, respective firstpolarization states; simulating receiving returning radiation from thepixels in response to each of the scans; simulating analysis of at leastpart of the returning radiation from the scans using respective secondpolarization states to generate processed returning radiation from thereturning radiation; simulating detection of the processed returningradiation; for each scan, constructing respective block signatures ofthe blocks in response to the processed returning radiation from thegroup of pixels in each block; for each scan, determining a blocksignature variation using the block signatures of the blocks; and inresponse to the block signature variation, selecting one of the firstpolarization states and at least one of the second polarization statesfor use in subsequent physical examination of a test object.
 18. Themethod according to claim 17, wherein determining the block signaturevariation comprises determining a highest block signature variation, andwherein selecting one of the first polarization states and at least oneof the second polarization states comprises selecting the first andsecond polarization states for the scan having the highest blocksignature variation.
 19. A method for characterizing a surface of asample object, the method comprising: simulating the surface as asimulated surface, and defining simulated characteristics of thesimulated surface; dividing the simulated surface into pixels which arecharacterized by a parameter variation; defining blocks of the simulatedsurface as respective groups of the pixels; simulating irradiation ofthe pixels in multiple scans over the simulated surface with radiationhaving different, respective first polarization states; simulatingreceiving returning radiation from the pixels in response to each of thescans; simulating analysis of at least part of the returning radiationfrom the scans using respective second polarization states to generateprocessed returning radiation from the returning radiation; simulatingdetection of the processed returning radiation; constructing respectivecomposite block signatures of the blocks in response to the processedreturning radiation from the group of pixels in each block; determininga composite block signature variation using the block signatures of theblocks; and in response to the composite block signature variation,selecting two or more of the first polarization states and at least oneof the second polarization states for use in subsequent physicalexamination of a test object.
 20. The method according to claim 19,wherein determining the composite block signature variation comprisesdetermining a highest composite block signature variation, and whereinselecting the two or more of the first polarization states and at leastone of the second polarization states comprises selecting the first andsecond polarization states corresponding to the scans forming thehighest composite block signature variation.
 21. A method forcharacterizing a surface, comprising: providing a sample object having avariation of a parameter; dividing the surface into pixels; definingblocks of the surface as respective groups of the pixels; irradiatingthe pixels in multiple scans over the surface with radiation havingdifferent, respective first polarization states; for each scan:detecting returning radiation from the pixels by one or more detectors,wherein for at least one of the detectors, at least part of thereturning radiation is passed through a polarization element configuredto pass radiation having a second polarization state different from therespective first polarization states; constructing respective blocksignatures of the blocks in response to the returning radiation;determining a block signature variation using the respective blocksignatures of the blocks, thereby generating a collection of blocksignature variations; and selecting at least one block signaturevariation from among said collection to thereby provide the respectiveone of the first polarization states and at least one of the secondpolarization states for use in subsequent characterization of a surfaceof a target object.